In order to investigate the significance of the age-dependent increase in the level of thymus chromosomal protease and the direct binding of the protease to the DNA only in thymus, we propose studies on: (a) the localization of the chromosomal protease in various thymus cells by autoradiography, cell fractionation, and fractionation of hydrocortisone-resistant cells, Con A stimulated cells, and the cells surviving X-ray irradiation; (b) the purification and characterization of the thymus chromosomal protease, the specificity of the protease toward substrates and inhibitors, and binding of thymus DNA during aging; (c) the possible effect of the protease on the nucleolytic degradation of thymus DNA during aging; (d) the isolation of the DNA sequences bound by the chromosomal protease and analysis of the DNA reassociation kinetics and hybridization of thymus mRNA to the DNA sequences bound by the protease; and (e) the possible age-dependent change in the DNA sequences bound by the protease.